This invention relates to electron guns and, more particularly, to an axial-type electron gun for generating an electron beam having very high power and high power density, applicable in large-scale vacuum metallurgy plants for melting, vaporizing and heat treatment.
Axial-type electron guns are known which generate electron beams and have nominal powers ranging from 60 kW to 1,200 kW. For improving beam generating reliability, it has been proposed in German Democratic Republic Pat. No. 112,879 to design a focusing electrode of an electron gun in such a manner that the potential of the focusing electrode is equal to, or smaller than, the potential of the filament used to indirectly heat a solid cathode of the electron gun. Furthermore, it has been disclosed in German Democratic Republic Pat. No. 132,380 that high power electron beams can be guided with improved quality by utilizing gas focusing.
It has, moreover, become known that a high power electron gun has, to some extent, a good operating condition if its electron beam is generated as a result of space charge limited emission of its cathode. More specifically, with sufficient cathode heating, the power of the electron beam can be controlled by varying the acceleration voltage. However, if the acceleration voltage is varied, all electric and magnetic fields necessary for guiding and positioning the electron beam must be readjusted accordingly. A technique for eliminating this drawback has been proposed in German Democratic Republic Pat. No. 134,168 and utilizes an axially adjustable anode, the adjustment of which controls electron beam power notwithstanding a constant acceleration voltage and a constant cathode temperature.
Enroute to its target, the electron beam of an electron gun typically passes, inter alia, an anode and a subsequent magnetic lens (beam guidance lens). In an electron-optical sense, the anode passageway represents a dispersion lens, but the beam guidance lens corresponds to a condensing lens. The expert knows and can prove by the electron-optical laws of imagery that the generally realized combination of an axially adjustable anode with a fixed beam guidance lens has both advantages and disadvantages. In this case, it is advantageous that a definite focal length of the fixed beam guidance lens can be chosen independently of the adjusted position of the anode, whereas it is disadvantageous that the aperture of the electron beam increases with increasing cathode-to-anode distance. Furthermore, the expert knows and can prove by the electron-optical laws of imagery that a beam guidance lens moved together with an axially adjustable anode has advantages and disadvantages, too. In this case, it is advantageous that the aperture of the electron beam decreases with increasing cathode-to anode distance, whereas it is disadvantageous that the focal length of the beam guidance lens must be made greater for optimally concentrating the electron beam.